It is known to focus a camera lens at a hyperfocal distance of a scene in order that the resulting image exhibit a greater depth of field. The hyperfocal distance is that distance at which objects between half the hyperfocal distance and infinity are in focus. Though the term “in focus” depends for a particular viewer on that viewer's visual acuity, the term is objective and well defined in the photographic arts. Specifically, an object is in focus if it is within the depth of field. Objects at only one point along an optical axis are precisely in focus at the image plane (film or charge-coupled device CCD sensing element), given a certain lens arrangement. Objects in front of or behind that point, within a certain range, are considered to be in focus if they are sufficiently sharp. The span of those ranges in front of and behind that one precisely focused point is the depth of field. When the lens is focused such that the near edge of the depth of field is one half the total length to the focal point, the lens is focused at the hyperfocal distance.
Technically, depth of field is the region where the size of the circle of confusion is less than the resolution of the human eye. Circles with a diameter less than the circle of confusion will appear to be in focus. The circle of confusion is a term relating to the fuzziest a point can be and still be called “in focus”. The circle of confusion is often calculated as the largest circle on the film that will still be seen as a point when enlarged to 8″×10″ and viewed from a normal viewing distance (2-3 feet). Generally, about 0.0026 mm is considered an “average” circle of confusion for traditional, non-professional film imaging (i.e., non-professional camera and photographer, print film, photo-store processing).
Hyperfocal distance is schematically illustrated in FIG. 1. Where the camera is focused precisely on an object at the plane 11 (at the hyperfocal distance 10), all objects in the scene at a distance from half the hyperfocal distance 12 to infinity are considered to be in focus. Said another way, the near limit 14 of the depth of field when focused at the hyperfocal distance 10 occurs at one half 12 the hyperfocal distance, and that field extends to infinity. In FIG. 1, all tree trunks are in focus when the camera is focused at the hyperfocal distance 10, illustrated as the plane 11. Conversely, assume that the camera were focused at infinity (where light rays incoming to the camera lens may be considered parallel, typically about 20 feet and greater), such as schematically shown at the plane 16 in FIG. 1. The near limit of the depth of field is then further from the camera than for the hyperfocal case, and would lie for example at the plane 18 of FIG. 1. Since both depths of fields extend to infinity, focusing at the hyperfocal distance yields a greater depth of field than focusing at infinity. Hyperfocal distance is an important concept for maximizing depth of field in images of scenes that have both near and far field objects.
Certain other photographic parameters influence depth of field. Wider lenses exhibit shorter focal lengths and therefore deeper depths of field (e.g., an 18 mm lens will focus a deeper field than a 105 mm lens, all else being equal). Smaller aperture openings result in deeper fields (an f/16 aperture gives a greater depth of field than f/4 for the same lens, all else being equal).
U.S. Pat. No. 5,771,408, entitled “Fixed Focus Camera Shooting a Hyperfocal Distance” details a fixed focus camera arranged to mount an objective lens system and a diaphragm member on a common tube. The lens system provides fixed focus, and the diaphragm member has a rotating member that rotates to reduce light from the lens system. The tube and body are coupled so that rotation of the tube opens and closes the diaphragm member, and moves the lens system back and forth. When the diaphragm opening is reduced by rotating the tube, the objective lens system is moved forward to a position where the subject is captured at a hyperfocal distance. This enables the camera to perform shooting at a hyperfocal distance, whereby the minimum object distance for shooting can be shortened when the diaphragm opening is reduced.
The above patent describes a digital video camera in which the focal length is matched to the hyperfocal distance. However, in certain respects the image seen by a viewer is not the image captured by the camera. What is needed in the art is a method and apparatus to better match the image captured by a digital camera, including a digital video camera, with a derivative of that captured image that is actually displayed.